High bandwidth optical communications systems typically use continuous wave (“CW”) lasers, passing through an intensity modulator, to encode and transmit data. The light emitted by the laser has a very narrow frequency spread, characteristic of a coherent light source. The modulator is driven by high bandwidth electrical signals. These signals spread the frequency spectrum of the light to roughly twice the bandwidth of the modulating electrical signals. However, generating these high bandwidth electrical signals is expensive using conventional arrangements, and the resulting spread of the optical spectrum limits the spacing of separate frequency channels in a wavelength division multiplexed system.